Devices such as printers, copiers, and fax machines use a photoreceptor (also known as a photoconductor) having a photoreceptor charge transport layer. One type of photoreceptor is known as a photoreceptor drum (also know as a photoconductor drum). As the photoreceptor drum is used, the thickness of the photoreceptor charge transport layer is reduced and, at a certain thickness point, the photoreceptor charge transport layer fails. In view of this, manufacturers of photoreceptor drums generally provide a fixed interval setting to replace the photoreceptor drum in the device. This fixed setting is set by the manufacturer for an entire population of a particular type of photoreceptor drum and does not take into consideration the manner or environment in which a user actually uses the device having the photoreceptor drum. Replacing the photoreceptor drum at a fixed interval typically results in more frequent replacement of the photoreceptor drum than what is required for an individual use of the device.
Instead of replacing the photoreceptor drum at a fixed interval, it has been considered that in-situ determination of the photoreceptor charge transport layer thickness could be made and used to predict failure of that photoreceptor drum. Predicting failure of the photoreceptor charge transport layer on a photoreceptor by photoreceptor basis eliminates the need for replacing the photoreceptor drum at a predetermined interval. This enables a user to reduce the cost of operating a device having the photoreceptor drum by running each photoreceptor drum to a point at which the photoreceptor charge transport layer is just about to fail.
Some effort has been expended to enable in-situ determination of photoreceptor charge transport layer thickness for devices that use bias charged roll chargers. This effort is based on key characteristic behaviors of bias charged roll chargers, and in particular, the saturation of the photoreceptor voltage at the characteristic “knee” of the charge curve.
Many marking engines still use non-contact charging of the photoreceptor. One type of non-contact charging is scorotron charging, which uses corona discharge to generate ions that are directed to a surface of the photoreceptor charge transport layer. A scorotron usually includes coronode wires with a scorotron grid formed by a metal mesh or screen placed between the coronode wires and the surface of the photoreceptor charge transport layer. The scorotron grid is biased to a potential close to that desired at the surface of the photoreceptor charge transport layer. When the surface potential of the photoreceptor charge transport layer reaches the potential of the scorotron grid bias, the photoreceptor charging process ceases.
Unfortunately, the key characteristic behaviors of bias charged roll chargers are completely inapplicable for photoreceptor devices that use scorotron charging.